The widespread proliferation of wireless devices has given rise to an expectation among wireless customers for constant and reliable communication at nearly all times and in all places. Wireless carriers typically attempt to provide reliable wireless coverage to their customers by deploying a plurality of outdoor base transceiver stations (BTSs) over a defined coverage area. These BTSs are interconnected to form a network that transmits and receives wireless communication signals and thereby provides communication links to wireless devices in the defined coverage area. However, many areas frequented by wireless device users are partially or completely shielded from the signals transmitted by the BTSs. To improve wireless signal strength in these shielded areas, repeaters, distributed antenna systems, and similar communications systems have been developed that extend wireless signal coverage into areas where the RF signal penetration from BTSs is normally limited or absent. For example, these communications systems might be deployed to extend coverage inside buildings, underground parking garages, tunnels, shadowed areas that are behind mountains or other obstructions, underground train systems and/or train cars, as well as various other isolated or shielded areas.
These communications systems in their simplest form include a donor antenna and a coverage antenna coupled by a bi-directional amplifier. The donor antenna is located where it can receive signals from, and transmit signals to, one or more donor BTSs. The downlink signal received from the donor BTS is amplified by the bi-directional amplifier and coupled to the coverage antenna, which transmits the downlink signal into the extended coverage area. In the uplink direction, the coverage antenna receives signals transmitted by the wireless devices, which are in turn amplified by the bidirectional amplifier and transmitted back to the donor BTS through the donor antenna. In this way, the communication system provides both downlink and uplink coverage enhancement between the wireless device and donor BTS.
Ideally, the donor antenna is located where it has a direct line-of-sight path to the donor BTS to ensure the best possible signal path between the donor BTS and wireless device. However, extended coverage communication system installations are often constrained by geographic location and the available donor antenna mounting points of the extended coverage site. These constraints may preclude obtaining a direct line-of-sight path to the donor BTS. Moreover, in systems that repeat signals sourced from multiple commercial carriers, the donor antenna may be required to obtain signals from multiple BTSs in different locations. The problem of obtaining a clean signal may be further compounded in a mobile repeater system, such as found in a train, because the radio frequency environment is constantly changing. These additional constraints typically make obtaining a direct line-of-sight to the donor BTS difficult. Without a direct line-of-sight path to the donor BTS, the signal at the donor antenna is typically comprised of multiple reflected signals that arrive from different directions. These multiple signals generally have differing amplitudes and arrival times, so that the received signal suffers from multipath distortion. Interfering signals from other sources, such as a neighboring BTS operating in the same frequency band, may further distort and reduce the signal-to-noise ratio of the received signal as compared to a direct line-of-sight signal.
Multipath and other signal distortion and/or interference may result in channel fading, increased inter-symbol interference, and generally reduced signal quality. These reduced quality signals may cause reduced data rates, increased bit error rates, garbled speech, and may otherwise negatively affect link quality and the resulting wireless device user experience. Due to the changing nature of multipath and interfering signals, signal distortion also tends to vary with both time and frequency. Thus, a signal that provides a reliable communication link to the donor BTS at one moment in time may be subject to multipath distortion or other types of interference at another time. This uncertainty adds an additional layer of unreliability to the communication link. Because conventional extended coverage systems simply repeat the signal received at the donor antenna, any distortion or interference present at the donor antenna is repeated into the extended coverage area. Wireless devices in the extended coverage area are thereby affected by the distortion and/or interference present at the donor antenna.
Therefore, there is a need for communications systems and methods of extending wireless coverage that provide improved quality signals to wireless devices.